In recent years, in the field of coating materials, replacement of conventional solvent-borne coating materials in which pigments and polymer materials are dispersed in organic solvents with powder coating materials in which organic solvents are not used has been advanced, caused by enhancement of consciousness of environmental problems.
Generally, a powder coating material contains a thermal fluidity modifier for a powder coating material (hereinafter referred to as a thermal fluidity modifier), which improves the thermal fluidity of the powder coating material at the time of thermal curing, for improvement of the appearance of the resulting coat. The thermal fluidity of the powder coating material at the time of thermal curing can be improved by reduction of the melt viscosity of the powder coating material. Therefore, a low glass transition temperature-acrylic polymer which has a low glass transition temperature and is liquid at room temperature is usually used as the thermal fluidity modifier.
The thermal fluidity modifier being widely used today is one in which such a low glass transition temperature-acrylic polymer is formed into a powder by being adsorbed on fine silica particles to improve the handleability and anti-blocking property of the powder coating material.
However, there are problems such that transparency, gloss, and hardness of the resulting coating film are deteriorated when the thermal fluidity modifier adsorbed on silica particles is used for the powder coating material. Moreover, the thermal fluidity modifier including the low glass transition temperature-acrylic polymer is not necessarily satisfactory in handleability and anti-blocking property even when the low glass transition temperature-acrylic polymer is adsorbed on silica particles, because the silica particles easily causes blocking when they are left alone and are aggregated.
In addition, when a powder coating material is used, cleaning of devices and machines in a production step, such as kneading, pulverizing, and classifying, and in a coating step, is not easy. Moreover, there is a case where defects in a coating film such as generation of crawling and deterioration of leveling property and of gloss may occur even when a powder coating material is mixed with only a small amount of another powder coating material having a different thermal fluidity modifier. In other words, there is a drawback in that it is very difficult to change the properties, such as the color, of the powder coating material.
Consequently, in selecting a powder coating material, compatibility with the presently most widely used powder coating material, in which the thermal fluidity modifier including the low glass transition temperature-acrylic polymer is contained, is sometimes required.
To solve the above-mentioned problems, there is disclosed, in Patent Document 1, a powder coating material in which a surface modifier having a specific solubility parameter and a specific number average molecular weight is contained, so that the anti-blocking property of the powder coating material and the transparency and flatness of the resulting coating film are improved.
In addition, in Patent Document 2, there is disclosed a powder coating material in which a thermal fluidity modifier having a specific solubility parameter and being solid at 20° C. is contained, so that anti-blocking properties of the powder coating material and appearance of the resulting coating film are improved.
Further, in Patent Document 3, there is disclosed a thermal fluidity modifier including a polymer having isobornyl methacrylate units, a homopolymer of isobornyl methacrylate having a high glass transition temperature, and improvements in appearance of the resulting coating film and in adhesion of the resulting coating film with a finish coating material are designed.    Patent Document 1: Japanese Patent Application Laid-Open No. Hei 7-179,789    Patent Document 2: Japanese Patent Application Laid-Open No. 2000-355,676    Patent Document 3: Japanese Patent Application Laid-Open No. 2001-131,462